The 3rd Generation (3G) mobile communication systems adopt Code Division Multiple Access (CDMA) techniques and support multimedia services, thus have strong competitiveness. To maintain the competitiveness for long time, 3GPP has launched a Long Term Evolution (LTE) research program for 3G wireless interface techniques.
At present, the LTE system supports 2 types of frame structures. Among three international 3G standards, TD-SCDMA is the only one that adopts TDD manner. In the long-term evolution of the TD-SCDMA (LTE TDD), a preferred frame structure is the frame structure type 2 which is compatible with the TD-SCDMA system, as shown in FIG. 1. Each radio frame has a frame length of 10 ms and consists of two half-frames of length 5 ms each. Each half-frame consists of 7 service slots (denoted as 0-6) and 3 special slots, i.e. a Downlink Pilot Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Slot (UpPTS). A service slot is defined as a subframe. The subframe 0 and the DwPTS are always reserved for downlink transmission while the UpPTS and the subframe 1 are always reserved for uplink transmission.
The LTE TDD system is based on Orthogonal Frequency Division Multiplexing (OFDM) technique. Subcarrier spacing is 15 kHz, and correspondingly, the length of an OFDM symbol is 66.67 us. A Cyclic Prefix (CP) is added at the head of each OFDM symbol for the purpose of OFDM symbol timing during de-modulation. For unicast services and applications with restricted coverage area, normal CP with a length of 4.76 us is adopted. Thus, the length of the whole OFDM symbol will be 66.67 us+4.76 us≈71.4 us. While for multi-cell broadcast services and applications with large coverage areas, extended CP with a length of 16.66 us is adopted, in which case the length of the whole OFDM symbol will be 66.67 us+16.66 us≈88.3 us.
In TDD systems, guard periods are necessary at downlink-to-uplink switch-points to avoid interference between downlink slots and uplink slots. Therefore, as described in the above, guard periods are adopted as special slots in the structure of the radio frame in current LTE TDD systems. The length of the GP slot equals to the time taken by electromagnetic wave for traveling twice the radius of a cell, i.e., TGP=2*Rcell/C, where Rcell denotes the radius of a cell, C denotes the velocity of light (3*108 m/s).
In the frame structure shown in FIG. 1, the length of the GP slot is 75 us, under which the maximum coverage area is (75 us/2)×3×108 m/s=11.25 km. The current length of the GP slot may be modified to support different coverage areas. The GP slot may be designed in the following manner: vacate one or more consecutive uplink slots for forming a larger GP slot between a downlink slot and its subsequent uplink slot to support a larger coverage area. Specifically, three lengths for the GP slot are currently provided for a base station to choose from based on the area of the cell covered by the base station.
1. For a small size coverage area where the radius of the cell is smaller than 7.5 km, the frame structure with respect to restricted coverage area shown in FIG. 2 may be adopted. At this time, the length of the GP slot is 50 us and random access may be performed within the UpPTS.
2. For a medium size coverage area where the radius of the cell is larger than 7.5 km but smaller than 30 km, the length of the GP slot according to the TDD frame structure type 2 is not long enough. The GP slot and the UpPTS may be combined to form a new GP slot which has a length of 191.66 us and can support a coverage area about 29 km. In this situation, random access may be performed within TS1 or any subsequent uplink slot. The frame structure which supports the medium size coverage area according to the TDD frame structure type 2 is shown in FIG. 3.
3. For a large size coverage area where the radius of the cell is larger than 30 km, whole TS1 in the TDD frame structure type 2 is reserved and combined with the GP slot and the UpPTS to form a new GP slot with a length of 866.66 us which is long enough to support a coverage area over 100 km. In this situation, random access is performed within TS2 and its subsequent uplink slots. The frame structure which supports the large size coverage area according to the TDD frame structure type 2 is shown in FIG. 4.
The base station and the user device respectively store frame structures corresponding to the above three lengths of the GP slot. The base station may select one of them based on the coverage area of the base station and inform the user device of the length selected. During subsequent data transmission between the base station and the user device, the frame structure corresponding to the selected length of the GP slot is adopted for carrying data.
According to the above, the granularity for adjusting the length of the GP slot equals to the length of one uplink slot. Therefore, the difference between coverage areas of different levels is relatively large. When employed for data transmission, the above method cannot support different coverage areas flexibly, which leads to a waste of radio resources and low transmission efficiency. For example, to cover a cell whose radius is 50 km, the third kind of length for the GP slot and the corresponding frame structure will be adopted for data transmission. But in fact, a large part of the slot is wasted acting as guard period, which also affects the transmission efficiency.